The cell biology and biochemistry of Leishmania and Trypanosoma are investigated as models of intra- and extracellular parasitism, respectively. As all interactions between host and parasite occur at the level of the parasite surface membrane (SM) emphasis is placed on: 1) its integrated biochemical characterization and 2) defining its roles in parasite survival. About 65 kDa avidin-binding glycoprotein was identified in the SM of L. donovani (L. d.) promastigotes which could function in carboxylase reactions. The major carbohydrate constituents of both SM and released glycoproteins of L. d. were identified using Western blots. The kinetics of synthesis and secretion of soluble acid phosphtase (SAcP) by L. d. were determinend and its glycosylation events identified. SAcP was purified and its N-terminal amino acid sequence determined. Complement component C3 binds to the SM of L. d. predominantly as iC3b and most is released from cells as cleavage fragments presumably reuslting from parasite SM protease activity. A facilitated diffusion-mediated pentose transport system was characterized in L. d. The L. d. SM 3'-nucleotidase (3'-NT) was isolated and charcterized as a 43 kDa mannose-glycoprotein. Clones putatively encoding for 3'-NT and a SM gp-63 antigen were isolated from a L. d. genomic library. The L. d. SM 5'-nucleotidase was partially purified and putatively identified as a 72 kDa mannosylated glycoprotein. Two distinct proton-ATPases were characterized in the SM and mitochondrion of L. d., respectively. The L. d. SM D-glucose transporter was characterized as a 19 kDA mannosylated-glycoprotein. Further, a 55-62 kDa thyrotropin binding protein was demonstrated in the SM of L. d. The current results provide further functional characterization of parasite SM constituents which might prove useful as targets for chemotherapy and/or as agents for immunoprophylaxis.